Ch 29:Distributing Time and Frequency Data

Judah Levine, Time and Frequency Division, National Institute of Standards and Technology, USA

Chapter Overview:

This chapter describes a number of methods that can be used to distribute time and frequency information with the goal of calibrating or synchronizing a local clock. The primary focus will be on methods that use the signals from the US GPS satellites. However, the basic techniques are quite general and are not limited to any specific navigational satellite constellation or source of calibration data.

The discussions assumes that the user has a local clock or frequency standard, and that the purpose of receiving time and frequency information from a remote source is to improve the time accuracy or the frequency stability of the local device. The goal then is to combine the data from the remote reference with the reading of the local clock to produce a combination that incorporates the best features of both contributions and to realize a combination that is better than either one by itself. From this perspective, any discussion of distribution methods should include some understanding of the characteristics and limitations of real clocks and frequency standards, and these are discussed in the introduction.

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Chapter Table of Contents

1.Introduction and clock statistics

1.1 Physics of clocks and oscillators and the parameters that are used to describe them

1.2 Clock noise

1.3 Measurement noise

1.4 Analysis for pure white phase noise

1.5 White frequency noise

1.6 Frequency drift

1.7 The Fourier picture and flicker noise

1.8 The iterative model of clock behavior

1.9 Time domain statistics, the simple two-sample Allan Variance and its fancier relatives

1.10 Statistics in the frequency domain

1.11 Summary of clock statistics

2. Accuracy and traceability

2.1 Technical traceability

2.2 Legal traceability

3. Determining the time delay through the distribution channel

3.1 Estimating the delay from ancillary parameters

3.2 Physical common view

3.3 The melting pot method of common view

3.4 Physical common view and the melting pot method compared

3.5 Multipath reflections

4 Determining the delay through the antenna and the receiver

4.1 short baseline common view

4.2 Hardware calibration

5 Synchronization strategies

5.1 Do no harm

5.2 A simple strategy

5.3 Rapid error correction

5.4 Multiple satellite error detection

6 Illustrative data

6.1 Statistics of atomic clocks

6.2 Statistics of GPS time

7 Adjustment methods

7.1 Correcting the clock with a single step

7.2 Slewing adjustment rate

8 Bibliography